Cerebral autoregulation analysis The investigators will use TCD to determinate CA situation. A continuous monitoring of MCA velocity and simultaneous invasive arterial pressure will be done every 48-72 hours during admission and when the patient is under MV. The investigators will calculate the Mx and Sx coefficients, defined as Pearson correlation between the average of mean (Mx) and systolic (Sx) MCA velocity and average of the values of arterial pressure waveform every 3 minutes. The software used is DWL Compumedics Monitoring ® to measure MCA velocity and arterial pressure waveform every 1 millisecond. The monitoring test will take between 20 minutes and 1 hour time.